Consider an Al-4% Si alloy. Determine (a) if the alloy is hypoeutectic or hypereutectic, (b) the composition of the first solid to form during solidification, (c) the amounts and compositions of each phase at 578˚C, (d) the amounts and compositions of each phase at 576˚C, (e) the amounts and compositions of each microconstituent at 576˚C, and (f) the amounts and compositions of each phase at 25˚C.

(a) Hypoeutectic

(b) Alpha solid, aluminium

(c) 70% α , 30% β

(d) 97.6% α, 2.4% β

(e) 97.6% α, 2.4% β

(f) 97% α, 3% β

Explanation:

(a) The eutectic composition for Al Si alloy is 11.7 wt% silicon, therefore, an Al-4% Si alloy is hypoeutectic

(b) For the hypoeutectic alloy, aluminium, Al, is expected to form first, such that the aluminium content is reduced till the point it gets to the eutectic proportion of 11.7 wt% silicon

(c) At  578°C we have

% α:  Al      (11 - 4)/(11 - 1) = 70% α

% L:  Si      100 - 70 = 30% β

(d) At  576°C we have

α: 99.83% Si    (99.83 - 4)/(99.83- 1.65) = 97.6% α

β: 1.65% Si (4 - 1.65)/(99.83- 1.65) = 2.4% β

(e) Primary α: 1.65% α (99.83 - 4)/(99.83 - 1.65) = 97.6% α

Eutectic 4% Si  = 100 - 97.6 = 2.4% β

(f) At 25°C we have;

α%: (99.83 - 4)/(99.83 - 1) = 97% α

β%:   100 - 97 = 3% β.